In automotive power amplifier devices an offset detector function is normally embedded. Any DC output offset exceeding ±2V is signaled out no matter if there is an input signal or not. This abnormal offset, when there is no input signal, might occur as a consequence of initially defective or aged and worn-out input capacitors feeding a DC component to the input. The abnormal offset can put the speakers at risk of overheating, or damage the speakers directly.
In current applications of automotive audio power amplifiers, if an audio system is to protect the speakers in case of high output offset, an MCU (microprocessor) must first read the offset detector outputs from the amplifier, and put the amplifier in a mute mode if a high offset is detected. All these actions require extra MCU operations and are controlled outside of the power amplifier IC.
To read the information given by the power amplifier (for example in the TDA7563B quad power amplifier) the diagnosis has to be performed with a low-level output AC signal (or Vin=0). An MCU enables the offset detection by setting an I2C command IB1D<5>=1 and then reads the status of the amplifier after a selectable testing time. An excess offset greater than ±2V is signaled out if it is persistent throughout the assigned testing time. The MCU then can decide to turn off or mute the power amplifier, or just do nothing.
If the MCU ignores all the operations described above, the power amplifier may maintain play mode with a high offset feeding to the speakers without any protections, which is undesirable for the reasons given above.
Referring now to FIG. 1, a flow chart 100 shows the turn-on sequence for the prior art amplifier described above. An MCU enables the offset detection, and sets the IBID<5> bit to a logic one at step 102. The MCU then reads the offset status at step 104. A decision block 106 asks whether or not the offset read is greater than two volts. If no, then the audio amplifier maintains its current working state at step 108. If yes, then the MCU sends a mute command to the amplifier, or simply turns the amplifier off at step 110.
Referring now to FIG. 2, a plot of the voltage at the two speaker output terminals 202 and 204 is shown in a turn-on procedure in the case of an abnormal offset without MCU intervention according to the prior art. The step function in traces 202 and 204 are caused by the abnormal offset in the speaker system.
What is desired is a more direct method and corresponding circuit apparatus for providing the prior art functionality more quickly, and without incurring the extra MCU cycles described above.